28 THE STRUCTURE OF 



sort, for tearing minute structures in pieces, will 

 be found very useful. 



When opaque objects are to be examined, the 

 light from the mirror may be shut off, and the aid 

 of the bull's-eye condenser called in. The object 

 being secured in the forceps attached to the stage 

 (fig. 15), or laid upon a slide, the| light is allowed 

 to fall on it through the condenser. (Fig. 9.) The 

 object-glass must be focussed in the same manner 

 as for transparent objects, till the best distance is 

 secured for examining it. The petals of plants, 

 the wings and other parts of insects, with 

 many other objects, can only be examined in 

 this way. 



Fig. 15. Stage Forceps. 



Even the beginner will find it useful to keep by 

 him some little bottles, containing certain chemical 

 re-agents. Thus, a solution of iodine is useful to 

 apply to the tissues of plants, for the purpose ot 

 ascertaining the presence of starch. This solution 

 may be made by adding five grains of iodine and 

 five grains of iodide of potassium to an ounce ot 

 distilled water. Strong sulphuric acid will be 

 found useful in rendering soff the tissues of both 

 plants and animals ; and in conjunction with iodine 

 it is a test for the presence of cellulose, the sub- 

 stance that forms the walls of the cells of plants. 

 In order to apply this test, the tissue should be 

 first touched with the sulphuric acid, and on the 

 applicatian afterwards of the solution of iodine 

 lihe blue colour of starch becomes evident. The 



